In mobile (battery powered) products, some system designers approach low battery operation by only allowing certain blocks and partitions of a system and/or amplifier to take up to a maximum current. This prevents brownout or reset of the main processor(s) due to the battery voltage collapsing on peaks. With mono-audio amplifier instantiations, a simple maximum current limit can be defined, reacting to audio peaks and providing graceful degradation to the audio material in the presence of peaks.
The problem of boosted amplifiers potentially triggering reset events at low battery is well established for mono-amplifiers. For example, some systems are designed to have an automatic level control (“ALC”) gain reduction block that is triggered by low battery conditions. This pulls down the gain of the amplifier, preventing any audio peaks from causing a reset. Also, at the system level, some attempts have been made to have the various partitioned blocks in a mobile device have a “pre-reset” bus warning i.e., an attempt to alert other devices that have asynchronous current demands to restrict current draw at key times.
The limitation of the prior art, for audio implementations, is that on larger (or tablet format) mobile devices where more than one amplifier is used, each amplifier has to make its own peak current limit decisions independently, even though the audio they are playing is most likely coherent in nature (for example, in stereo replay). Any attempt of one channel to limit audio peaks, based on the local version of battery voltage and program material, is likely to degrade or eliminate stereo imaging or otherwise degrade the audible experience, since the limiting behavior of other channels is unknown.
As noted above, due to the distributed nature of power and GND planes, as well non-idealities in determining battery voltage, different amplifiers in a system may well experience different battery supply voltages. For example, if one channel is on the verge of detecting a low battery event, another channel may already switch to a “protect mode,” due to tolerances and/or local IxR drops. This makes dealing with the problem of low battery operation in such systems difficult.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.